1. Field of the Invention
This invention relates to displays, more specifically to High Definition Displays (HDD) or High Definition Television (HDTV) displays.
2. Background of the Invention
HDTV has pointed out problems with several current technologies in the display area as these technologies attempt to adapt to the demands of the new standards. The main types of display systems are cathode-ray tubes (CRT), and liquid crystal displays (LCD). Each of these have weaknesses that HDTV requirements make more obvious.
CRT
The cathode-ray tube display is probably the most common type of display, with worldwide acceptance as televisions, computer monitors and many different display tasks. CRTs typically function with an electron gun or guns that sweep from side to side across the back side of a piece of glass that has been treated with various phosphors. The phosphors glow when the energy from a gun touches them, and creates the images on the screen. The current U.S. TV standard (NTSC) displays images across approximately 480 horizontal lines. The image data is interleaved, meaning that the odd-numbered lines are displayed in one time interval and the even in another. (The current European standard is similar, having approximately 576 horizontal lines.) In the NTSC standard the time interval is 16.7 milliseconds. This time allows the gun to "dwell" on any particular pixel spot on the screen for a finite amount of time.
The times and line counts mentioned above are for current, standard resolution television. However, some HDTV and EDTV (enhanced definition television) standards are not interleaved, but progressively scanned. This means that the data is displayed in line order as 1, 2, 3 . . . etc. Therefore, in the time formerly available to display 240 lines, EDTV and HDTV must display 480 or 960 lines respectively. In this situation the pixel dwell time has dropped by a factor of two or four respectively, lowering the brightness of each pixel, since it will receive less energy. Additionally, HDTV has an aspect ratio of 16:9 instead of 4:3, making the lines even longer, resulting in less time per pixel.
Therefore, the use of a CRT in HDTV might not be satisfactory. Unless some sort of compensation is made, the brightness will drop by at least a factor of four. Since CRTs are a mature technology, there is little chance that a massive industry effort can achieve the brightness levels of standard CRTs in HDTV displays. The resulting HDTV display will compare very poorly to standard television sets in brightness, and consumer acceptance would be poor. For reasons such as those discussed above, CRTs do not appear to be a truly feasible choice for HDTV, and one must tun to LCDs.
LCDs
Liquid crystal displays have a large advantage because they have individually addressable cells. These individual cells become individually controllable picture elements, or pixels, in the display environment. To adapt to HDTV, the number of cells must increase, but there is not necessarily any loss of brightness associated with this pixel increase. However, LCDs have other interrelated problems. First, they have low manufacturing yields and therefore are expensive. Second, they have low transmission efficiency. Third and fourth, they have long response times and low contrast ratios.
Several factors contribute to high LCD costs. The biggest factor is low yield in the fabrication steps. The pixel addressing transistors must be made on a glass substrate. Typically, these substrates are large and difficult to handle and usually cannot share standard silicon processing techniques or use standard silicon processing equipment. Therefore, the fabrication process cannot benefit from the economy of scale of the silicon processing industry. Complete LCDs require intricate physical assembly and again cannot share the equivalent silicon packaging process. Additionally, LCD materials are expensive in and of themselves.
LCDs typically consist of two glass panels with the crystalline material sandwiched between them. One or both of the glass panels have transistors functioning as electrodes that activate the crystalline material. When the electrode is activated, the crystals, which are aligned along a polar axis, twist, layer by layer. The effect is that the light entering through a polarizer on one side of the LCD travels through the twisted crystals and is absorbed in a second polarizer, causing dark pixels. The light must be polarized before entering the crystal matrix. Typically, a polarizing coating on the front panel of the display accomplishes this. Absorption in these two polarizers typically accounts for a loss of over 60% of the incident illumination, making LCDs inefficient.
The process of twisting crystals is in itself a problem. The response time of the crystals to the electrode signals is slow, on the order of 30 milliseconds. As mentioned above, HDTV will require faster, not slower, response times in the display because of the higher amount of information required for the display. Since the LCD relies upon the extinction of transmitted light within the display itself, heating of the device through absorbed radiation is a concern. Also LCDs have low percentage active areas that lead to further inefficiency.
Because of the high cost, low efficiency, slow response time and low contrast ratio, LCDs are undesirable for HDTV. HDTV must be accessible to a broad spectrum of consumers, and must produce a noticeable difference from the present day television sets. Such an advantage will be difficult for LCDs and CRTs to achieve.
Therefore, HDTV requires a display technology that has good brightness, low cost, ease of manufacturing and flexibility to adjust to different standards of television broadcast formats.